84 



THE MEXICAN COTTON-BOLL WEEVIL. 



application of Dr. Marchal's law to the abundance of the boll weevil 

 will be discussed in the folio wi'ng paragraphs. 



When the boll weevil entered the United States, it was released 

 from most of its natural enemies and was in the portion of the cotton 

 belt most resembling its natural home. Naturally, it increased with 

 great rapidity. In fact, the weevil was on what may be called the 

 upward curve of numerical abundance from 1892 to 1896. In the 

 meanwhile, native parasites began to adapt themselves to it, and we 

 may assume that their abundance might be indicated by a curve par- 

 allel to but behind that of the boll weevil. In 1896 a severe drought 

 was the cause of a very sudden decrease in the numbers of the weevil 

 and of course also acted upon the parasites. Following 1S96 the 

 increase in abunclance of the weevil was comparatively slow, owing to 

 the unlimited opportunities for spread. The maximum point in this 

 increase appears to have been reached in the autumn of 1904 and 

 may have Tbeen partly due to the fact that in that year the abundance 

 of the parasites was on the decrease. In the winter of 1904 a severe 

 cold period turned the curve of abundance downward, but the decrease 

 was slow until the fall of 1907, when another severe freeze caused a 



sudden falling off. 

 Floods in the spring 

 of 1908, drought in 

 the summer, a freeze 

 in the fall of the 

 same year, and 

 droughts in the sum- 

 mers of 1909, 1910, 

 and 1911, followed 

 in 1909 and 1910 bv 

 severe winters, all 

 combined to reduce 

 the weevil still more. 

 On the other hand, 

 from 1904 to 1908 

 the influence of the 

 parasites was in- 

 creasing and from then until 1911 decreasing. As Dr. Marchal 

 pointed out, it is very rare that some condition does not intervene 

 just before the number present has reached zero and save the species 

 from extermination. The weevil will undoubtedly frequently be 

 greatly reduced in large regions, but in such areas the inflow from 

 other localities will serve to bring about earlv reestablishment. (See 

 %. 20.) 



Undoubtedly the adverse seasons of recent years will be followed 

 by others which will allow the weevil to reach approximately its 

 former abundance. This alternation of years of scarcity and of 

 abundance will continue indefinitely. Naturally, no definite predic- 

 tion can be made as to the number of years which will be included in 

 the alternating periods. 



The series of Texas maps presented herewith (figs. 15-19) illus- 

 trates the variations in the percentage of infestation in August during 

 a series of years in which the weevil abundance was at a low ebb. 

 They also show very plainly how the areas of heavy damage are 

 shifted by more or less local causes. 



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Fig. 20.— Curves of numerical strength of the boll. weevil and its para- 

 sites. The boll weevil curve is at the scale of 2 to 10 and represents 

 the percentage of infestation in August in Texas. The parasite curve 

 is at the scale of 1 to 1 and represents the percentage of mortality of 

 the boll weevil due to parasitism. (Original.) 



